memcg: cleanup root memcg checks

Replace the implicit checking of root memcg with explicit root memcg
checking i.e.  !css->parent with mem_cgroup_is_root().

Link: https://lkml.kernel.org/r/20210223205625.2792891-1-shakeelb@google.com


Signed-off-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>